1. Field of the Invention
The present invention relates to a spread illuminating apparatus, and particularly to a spread illuminating apparatus used as an illuminating means for a liquid crystal display.
2. Description of the Related Art
A liquid crystal display characterized by its small occupied volume, low-profile and light-weight has been extensively used in electric products including cellular phones and personal computers and the demand thereof has been increasing. However, since a liquid crystal of the liquid crystal display (hereinafter, referred to as xe2x80x9cLCDxe2x80x9d) does not emit light by itself, a separate illuminating means for irradiating the LCD is needed besides the LCD when used in dark places where sunlight or room light is not satisfactorily available. Thus, this illuminating means is required not only to be small in size and in power consumption, but also to project high quality images on an observation surface. In particular in recent years, a sheet-like spread illuminating apparatus of side light type (light conductive plate type) is often applied as an illuminating means.
FIG. 5 shows an embodiment of a conventional spread illuminating apparatus of side light type, which aimed for a uniform spread light emission (disclosed in the Japanese Unexamined Patent Application Publication No. 2000-11723) described hereinafter.
A spread illuminating apparatus 1xe2x80x2 disposed over an upper surface of a reflection type liquid crystal element L is generally composed of a flat rectangular light conductive plate 2 made of a light-transmissible material, a lamp 4 disposed close to a side surface 3 thereof, and a light reflection member 12 (a reflector) covering the lamp 4 and two edges of top and bottom surfaces of one end of the light conductive plate 2.
The lamp 4 is composed of a light conductive bar 7 and light sources (shaped spot-like) 9 and 9 such as light emitting diodes disposed at both end surfaces 8 and 8 of the light conductive bar 7. An optical path conversion means 11 including grooves 15 substantially triangular in section and flat portions 16 formed therebetween is formed on a side surface 14 opposite to a surface 13 facing the side surface 3 of the light conductive plate 2. Light traveling inside the light conductive bar 7 tends to be reflected mostly by means of each inclined surface forming each of the grooves 15 so as to advance in the direction substantially perpendicular to the surface 13. Comparing with the flat portions 16, the grooves 15 reflect more amount of light entering the light conductive plate 2 after passing through the side surface 13. Thus, the ratio of the width of each of the grooves 15 to the width of each of the flat portions 16 is set to be proportional to the distance from each of the ends 8 of the light conductive bar 7. Since the width of each of the grooves 15 in the optical path conversion means 11 is formed in consideration of the distance from the light source 9, the uniform emission of light from the surface 13 can be realized regardless of the fact that the light sources are each disposed at each of the end surfaces 8, 8 of the light conductive bar 7.
In this connection, the configuration of the optical path conversion means 11 is not limited to the above embodiment, and the optical path conversion means 11 may comprise light scattering portions with minute ruggedness formed by roughening the surface thereof and flat portions without ruggedness.
A light reflection pattern 17 is formed on an upper surface 6 of the light conductive plate 2 in parallel to the side surface 3. The light reflection pattern 17 comprises a plurality of grooves 18 each substantially triangular in section and flat portions 19 adjacent thereto, and the grooves 18 is spaced unevenly in order to realize the uniform spread light emission of the light conductive plate 2 irrespective of the distance from the lamp 4. This means that the ratio of the width (occupied area) of each of the grooves 18 to the width (occupied area) of each of the flat portions 19 is set to be proportional to the distance from the end surface 3 of the light conductive plate 2.
The light reflection member 12 covers longitudinal surfaces of the light conductive bar 7 except the surface 13 facing the light conductive plate 2, and also covers two edges of upper and lower surfaces of the one end of the light conductive plate 2 which is close to the light conductive bar 7 in order to recover light leaking out of the conductive bar 7 and to make an efficient utilization of the light traveling within the light conductive bar 7. The light reflection member 12 formed substantially U-shaped has, on its surfaces covering the light conductive bar 7 (inner surfaces), any one of a film on which a metal such as silver is vapor-deposited, a hard resin with a white film adhered to its inner surface, and a bent metal sheet such as a bent aluminum sheet, and a bent stainless steel sheet.
However, in the spread illuminating apparatus with the above configuration, there is a shortcoming that light-and-dark stripes are generated in the direction orthogonal to the side surface 3 of the light conductive plate 2 when observing the screen, which is due to the optical path conversion means 11 formed on the light conductive bar 7. That is, most of light reflected by the light scattering portions 15 of the optical path conversion means 11 enters the light conductive plate 2 after being emitted from the side surface 13, whereas most of light reflected by the flat portions 16 is totally reflected and travels within the light conductive bar 7 without being emitted from the side surface 13. Accordingly, it is, in a strict sense, impossible to make a luminous intensity at the side surface 13 become uniform due to the above design pattern comprising the light scattering portions 15 and the flat portions 16. As a result, the lightness of the light entering the light conductive plate 2 becomes non-uniform, and the light-and-dark stripes orthogonal to the end surface 3 are generated on the observational screen. One of the countermeasures therefor is to make the light scattering portions 15 and the flat portions 16 more minute to the level that the light-and-dark stripes can not be visually recognized. However, it is difficult to obtain a desired machining accuracy by employing this method.
Another countermeasure therefor is that, as shown in FIG. 6, a diffusion plate 20 is interposed between the light conductive bar 7 and the light conductive plate 2 so as to make uniform the luminance of the light entering the light conductive plate 2. In the diffusion plate 20 a light diffusion unit containing a light diffusive substance is formed on a plate-like supporting base. The light emitted from the side surface 13 of the light conductive bar 7 is diffused when passing through the diffusion plate 20 so that the luminance of the light entering the side surface 3 of the light conductive plate 2 can be made substantially uniform (disclosed in the Japanese Unexamined Patent Application Publication No. 2000-231814).
A spread illuminating apparatus 1xe2x80x3 with the diffusion plate 20 shown in FIG. 6 is effective for making the luminous intensity uniform on an observation surface. However, the light emitted from the side surface 13 and diffused during passing through the diffusion plate 20 has a lower light transmissivity for entering the side surface 3 of the light conductive plate 2, so that more power consumption is needed to obtain the predetermined luminance on the observation screen. Moreover, due to the complicated manufacturing steps, the working efficiency will be accordingly decreased.
In addition, even if the optical path conversion means is spaced as narrowly as possible in the hope of obtaining the desired machining accuracy without using any diffusion plate, it is almost impossible to prevent the light-and-dark stripes from being generated on the screen.
Although the details are given later, light-and-dark stripes having longer pitches than the aforementioned stripes are also observed so that countermeasures therefor are also necessary.
The present invention has been made in light of the above problems, and it is an object of the present invention to provide a spread illuminating apparatus which can easily obtain the uniform spread lightness over the entire screen without increasing the power consumption.
In order to solve the above problems, according to a first aspect of the present invention, a spread illuminating apparatus comprises: a light conductive plate which is made of a light-transmissible material and which has a light reflection pattern formed on a surface thereof; a lamp which comprises at least two light conductive bars arranged in parallel with each other and disposed along and close to a side surface of the light conductive plate, and a spot-like light source disposed over respective one end surfaces of the light conductive bars; and optical path conversion means each formed on side surface of each of the light conductive bars opposite to a side surface facing the side surface of the light conductive plate and adapted to reflect light emitted from the light source. In this structure, the optical path conversion means are arranged such that light-and-dark striping to appear on the light conductive plate due to light reflected by one optical path conversion means of one light conductive bar of the at least two is corrected by light reflected by the other optical path conversion means of the other light conductive bar.
In accordance with the present invention, light emitted from the light source is reflected toward the light conductive plate by the optical path conversion means formed on one end surface of each light conductive bar. Light-and-dark striping appears on the light conductive plate due to light reflected by the one optical path conversion means of the one light conductive bar. The light-and-dark striping is corrected by the other optical path conversion means of the other light conductive bar such that dark portions of the striping generated by the one optical path conversion means is matched with light portions generated by light reflected by the other optical path conversion means, and vice versa, whereby uniform spread light emission over the entire screen can be achieved.
And, according to a second aspect of the present invention, in the first aspect, the optical path conversion means each comprise a plurality of grooves and a plurality of flat portions adjacent thereto. Thus, light emitted from the light source is not only efficiently reflected toward the light conductive plate, but also reflected in a substantially uniform manner irrespective of the distance from the spot-like light source.
Further, according to a third aspect of the present invention, in the first aspect, the optical path conversion means each comprise a plurality of grooves shaped triangular in section and arranged continuously so that light emitted from the light source is not only efficiently reflected toward the light conductive plate, but also reflected in a substantially uniform manner irrespective of the distance from the light source.
Furthermore, according to a fourth aspect of the present invention, in the second or third aspect, the other optical path conversion means has one of, or a combination of the following three configurations: the grooves are formed only at positions corresponding to dark portions of the light-and-dark striping to appear on the light conductive plate due to the one optical path conversion means; the grooves have an increased depth at positions corresponding to dark portions; and the grooves have a decreased pitch at positions corresponding to the dark portions. With these configurations, the amount of light reflected toward the light conductive plate can be increased by the other optical conversion means at the dark portions generated by the one optical conversion means, thereby preventing the light-and-dark striping on the light conductive plate.
Still further, according to a fifth aspect of the present invention, in any one of the first to fourth aspects, the light source is disposed on one or both end surfaces of the light conductive bars disposed in parallel with each other so as to cover the end surfaces of all of the light conductive bars. Light emitted from the light source can efficiently enter each of the light conductive bars by setting the sum of the widths of all of the light conductive bars to be substantially equal to the width of the light source so that the light source can confront entirely the end surfaces of all of the light conductive bars.
Finally, according to a sixth aspect of the present invention, in any one of the first to fifth aspects, the one light conductive bar has a larger width than the other light conductive bar. The light-and-dark striping to otherwise appear on the light conductive plate can be corrected by appropriately setting the width of the other light conductive bar considering a balance with the amount of light reflected by the one light conductive bar generating the light-and-dark striping.